A lockable switch mechanism is described in European Patent Application No. 1376632. That mechanism comprises a switch plunger which is mounted in a housing and is displaceable relative to the housing along a predetermined axis between a first unlocked position and a second locked position. A locking mechanism is provided for locking the switch plunger in the second position and the switch plunger actuates a switch mechanism as a result of movement of the switch plunger between the first and second positions. The locking mechanism comprises two rolling element latches which are normally biased against the switch plunger so as to engage behind an axially facing surface defined by the switch plunger when the plunger has been moved to the second position. The rolling element latches can only be withdrawn so as to permit axial displacement of the switch plunger if a locking plate extending transversely of the switch plunger and having limbs extending in the axial direction of the switch plunger and engagable with a respective one of the rolling element latches is displaced to a latch release position. The latch releasing plate is driven directly by a solenoid which is disposed axially at one end of the switch mechanism housing. The arrangement is such that the switch will hold in its locked position despite the application of a considerable physical force in the opening direction. One particular design is arranged to withstand the application of a holding force of 1600N. It would require the effort of a stronger than average person to apply such an opening force. Hence the mechanism affords a latching mechanism that, prevents unintentional opening—such as is required for latches of safety gate for kinetic machines. However, a disadvantage of the known construction which arises from the relatively ductile material from which the locking element is made is that if the mechanism is loaded to such levels in its closed position it is possible for the locking element to deform without releasing the switch plunger and without tripping the associated electrical safety switch.
The consequence of this is that the holding force of the switch mechanism maybe considerably reduced. Thus a situation can arise where the mechanism has been subject to its indicated maximum load resistance but without providing any indication that this is the case. This may be disadvantageous in certain applications.
It is an object of the present invention to provide an improved lockable switch mechanism.